It is well known among medical practitioners that a patient with a cancerous tumor can be treated successfully by a process which raises the temperature of the tumor. This treatment is generally referred to as hyperthermia. Remission of a tumor can be affected by elevating its temperature to 40.degree..ltoreq.T.ltoreq.44.degree. C. It is desirable to provide for a uniform temperature distribution within the treated tissue.
One method of hyperthermia treatment is the use of electromagnetic radiation energy. The temperature of the tissue irradiated by the energy is a function of the power or intensity of the signal applied to the surface of the body tissue. The depth of penetration into the body is an inverse function of the signal frequency employed. The volume of the tissue to be treated is controlled by the electrical and geometrical design of the signal applicator. It is known that a flexible applicator can be utilized to conform to irregular surfaces.
Electromagnetic applicators radiate waves which propagate at the speed of light, 300,000 km/sec in a vacuum, or slower in matter. Such waves are characterized by both a propagation direction and a vector polarization.
Prior methods of hyperthermia treatment employ a waveguide applicator to supply the signal for irradiation of the treated tissue. The distribution of the irradiating signal from the waveguide applicator is manifested as a pattern of standing waves of the operating frequency. The standing wave distribution produces maximum and minimum voltage points which develop non-uniform irradiating signals, correspondingly producing undesirable non-uniform heating of the treated tissue.
An optimal non-invasive applicator delivers maximum power to the tumor while minimally heating surrounding healthy tissue. Since waves attenuate, i.e., deposit power, as they penetrate lossy tissue, a focusing source arrangement is required. Constructive interference at the tumor is obtained by adjusting the phase and amplitude of each point of the signal source. For constructive interference at the focal point, the electrical field at the tissue surface must be properly aligned and phased so that waves propagating along all paths in the entire tissue volume arrive in the same fashion. Merely adjusting phase, polarization, and amplitude for maximum focusing, however, does not necessarily produce an acceptable power-density distribution. It remains a problem that certain areas receive significantly more energy than adjacent areas, i.e., non-uniform heating.
It is well known that waveguide transmission lines can efficiently conduct or transmit electromagnetic energy. Waveguides such as this that are useful for carrying electromagnetic energy in the frequency range below one gigahertz, however, are also generally large and cumbersome unless they are dielectrically loaded, or filled, with a dielectric material having a dielectric constant substantially greater than unity.
A further requirement of applicators is a capacity to monitor power being deposited in the exposed tissue. Applied dosage information may be used as an approximate substitute for the difficult problem of direct, non-invasive temperature measurement.
Specific prior art hyperthermia techniques and apparatus therefor include: U.S. Pat. No. 4,197,860 issued 15 Apr. 1980 to Sterzer; U.S. Pat. No. 4,633,875 issued 6 Jan. 1987 to Turner; and U.S. Pat. No. 4,934,365 issued 19 Jun. 1990 to Morgenthaler.
U.S. Pat. No. 4,179,860 discloses a hyperthermia applicator and feedback control using a completely independent, i.e., non-integrated, albeit non-invasive, radiometer for sensing temperature of the heated tissue. When the sensed temperature exceeds the upper bound of a desired range, heating is inhibited. Heating is restarted after the sensed temperature falls below the lower bound of the desired range. The antenna in the applicator is a flat array of printed dipoles that does not act as a coaxially structured antenna and thus fails to produce a transverse electromagnetic (TEM) mode. Further, the antenna suffers the problem of producing dispersed hot spots. Also, nothing is disclosed or suggested about regulating the temperature of the dielectric within the applicator.
U.S. Pat. No. 4,633,875 discloses a hyperthermia applicator of waveguide construction having feedback control, albeit employing invasive temperature sensors. Invasive techniques are not desirable because they increase the trauma to the patient and risk the mixing of abnormal cells (those being treated) into healthy tissue. Here also, the rectangular waveguide does not operate in the TEM mode.
U.S. Pat. No. 4,934,365 discloses a hyperthermia applicator of troughguide construction which also suffers the problem of dispersed hot spots. It merely suggests the possibility of electronic feedback control, stopping short of suggesting thermometry feedback (be it invasive or non-invasive). Here, the radiometry employed is not microwave radiometry nor is the TEM mode produced.
Other attempts have been made to use the same antenna structure for both transmitting/heating and radiometry purposes. But since radiometry is best suited to using multifrequencies ranging from 1 to 4 GHz and the applicator is best suited to using one frequency between 100 MHz and 1 GHz, such a dual-function antenna has to be very broadband. No one has had success with such an arrangement.
Thus, the prior art has failed to teach an applicator having incorporated therein a radiometric receiving antenna. Further, the prior art has failed to teach such a combined antennas producing a TEM mode having a principal, non-dispersed maxima.